Silicate glass composed primary of SiO.sub.2 is highly transparent and can easily be molded (deformed) at high temperatures. Sheets of silicate glass, which have been formed with holes or concavities and convexities by microscopic topographic processing, are widely used as glass substrates for optical components used for optical communications and display devices.
In order to make a hole in a sheet of silicate glass according to microscopic topographic processing, it has been the general practice to process the sheet of silicate glass with wet etching (chemical etching) using an etchant of hydrofluoric acid or the like, or dry etching (physical etching) such as reactive ion etching.
However, wet etching suffers problems with respect to management and processing of the etchant. Dry etching requires pieces of equipment such as a vacuum container, needs a large-scale apparatus, and is not efficient because a pattern mask has to be produced by complex photolithography.
Laser beams have an intensive energy, and have heretofore been used to increase the temperature of a surface of a material to which the laser beam is applied thereby to ablate or evaporate a portion of the material to which the laser beam is applied, for processing the material in various ways. Since the laser beam can be focused Unto a very small spot, it is suitable for microscopic topographic processing of a material.
Then, in Japanese Patent Laid-Open No. 54-28590 (1979), there is disclosed processing of a glass substrate surface by radiating it with a laser beam while moving a table in X-Y directions, on which table is fixedly mounted the glass substrate already heated to 300 through 700.degree. C.
Although, as mentioned above, the concavities and convexities of desired shape can be formed on the glass surface by moving the table in X-Y directions, the concavities and convexities cannot be created if it is for instance a microscopic pattern such as of a diffraction grating.
Moreover, the movement of the table generates dust, which results in defects in the products and decreases productivity thereof.
As another method of manufacturing a planar microlens array etc., a stamper method has been already known, in which lens material is injected into a mold frame and the molded patterns are transplanted on the glass substrate and baked, however, it requires accurate positioning during the pattern transplanting process and the baking process, and it takes time.
As another method of manufacturing a planar microlens array etc., it has been proposed to obtain a convex lens by forming concavities arc shaped in cross-section on the glass substrate surface with a wet etching and injecting plastic material of high refractive index into the formed concavities, thereby forming the convex lens with the concavities, however, the wet etching has the problems as mentioned above.
Then, it is conceivable to form the concavities into which the plastic of high refractive index is injected by radiating a laser beam through a mask, however, since the laser beam has tendency of going straightforward and it has almost same intensity within area of one spot after passing through the openings of the mask, then the wall of the concavity formed on the glass substrate comes to be about perpendicular to the glass substrate, whereby it is impossible to obtain the cross-section of perfectly continuous arc shape. Therefore, it cannot be mounted onto apparatus requiring extremely high accuracy, such as a liquid crystal display, as it is, and it needs more or less treatment by wet etching and takes time.
Laser beams are generated by an infrared laser such as a CO.sub.2 laser, a Nd:YAG laser, a laser comprising a Nd:YAG laser combined with a wavelength conversion capability for producing a laser beam whose wavelength ranges from a near-infrared region through a visible region to an ultraviolet region, and an ultraviolet laser such as an excimer laser such as an ArF or KrF laser. If the CO.sub.2 laser of long wavelength is used, cracking due to thermal strain occurs violently. If the ultraviolet KrF laser (wavelength of 248 nm) is used, cracking occurs around the area where the laser beam is applied, therefore it is not suitable for the microscopic topographic processing. Thus, the use of the ArF excimer laser of wavelength of 193 nm is optimum as the laser beam for glass processing, however, even when such an ArF excimer laser is used, because of absorption by air, it is needed to replace the air with absorption-free gas such as Ar, etc. or to keep a vacuum in order to allow the laser beam to reach as far away as possible.